This invention relates to a semiconductor device and method of fabricating the same, and more particularly to a monolithic integrated semiconductor device of a semiconductor laser and optical waveguide and a method of fabricating the same.
Along with the recent development in the field of optical communications and optical information processing, there is a mounting demand for a semiconductor device in such a construction so as to couple the output light of a semiconductor laser with the optical waveguide at high efficiency. Principal applications of this semiconductor device are, firstly, the use in a so-called monolithic OEIC (opto-electronics integrated circuit) which guides the output light from the semiconductor laser in the same semiconductor substrate as the semiconductor laser, and performs such operations as modulation, switching and opto-electric conversion, and, secondly, the use in a so-called optical feedback laser which guides the output light from the semiconductor laser into the optical waveguide and feeds back to the semiconductor laser so as to stably unify the oscillation wavelength of the semiconductor laser and reduce the spectral linewidth.
Conventionally, as the coupling method of this kind of semiconductor laser and optical waveguide on a same semiconductor substrate, a method called a butt-joint process has been widely employed. (For example, it was reported at the General Meeting of the Institute of Electronics and Communication Engineers of Japan in 1982, Proceedings 4-42.)
In this conventional butt-joint semiconductor device, since the active layer and optical waveguide layer of the semiconductor laser were fabricated by different epitaxial crystal growths, that is, by performing crystal growth twice, the process was inevitably accompanied by a uneven film thickness or a film thickness control error due to etching controllability or an epitaxial crystal growth operation, bringing about axial deviation of the light guided between the active layer and optical waveguide layer, and the light coupling efficiency was notably decreased, making it difficult to achieve the intended characteristics. Besides, the thus fabricated semiconductor devices were not uniform in quality, which was a serious industrial problem. Concerning such significant decrease of light coupling efficiency due to axial deviation, for example, results of analysis were reported at the General Meeting of the Institute of Electronics And Communication Engineers of Japan in 1982, in Proceedings 4-43, in which it was disclosed that the coupling efficiency is extremely lowered by a very slight axial deviation, film thickness unevenness, or film thickness control error of even less than 1 .mu.m.